Measuring devices for characterizing microfluidic two-phase flows are known.
The expression “two-phase flow” is understood to mean a flow of a first fluid in another fluid, the fluids being immiscible. In this context, liquid/liquid flow may for example be provided in which the two liquids are immiscible. It is also possible in this context to provide a gas/liquid flow.
In channels such as micro channels, a two-phase flow takes the form of a liquid jet of a first fluid in another fluid or the form of a succession of droplets of the first fluid in the other fluid. In the intended application, of seeking to obtain a flow of droplets, the characterization of the two-phase flow then consists in characterizing the droplets of the first fluid by providing for example the length of each droplet, their velocity or else the transit frequency of the droplets. This remains the case in milli channels, for which the flows however have higher flow rates.
To characterize a two-phase flow of droplets or bubbles in a micro channel, certain techniques employ one or more laser beams passing through the micro channel. For example, with a laser beam and in the case of a liquid/liquid flow, it is possible to determine the frequency of emission of droplets of a first liquid in the flow of the other liquid. Moreover, with two laser beams illuminating the micro channel at two different places along the latter, it is also possible to determine the average flow velocity of the droplets of the first liquid in the micro channel.
However, the known devices using a laser beam do not allow the size of the droplets to be determined, the measurement of this size remaining qualitative. Moreover, they can be used only for micro channels that are transparent to the laser beam.
The reader may take note of this technique in the article by W. Engl et al., “Droplet traffic at a simple junction at low capillary numbers”, Phys. Rev. Letters 95, 208304 (2005).
Other techniques employ methods based on the refractive properties of a droplet of a first liquid flowing in another liquid. For this purpose, such devices emit a light beam along the direction of the micro channel and provide a position detector on the other side of the channel. By knowing the deviation of the beam it is then possible to determine the emission frequency and the size of the droplets.
However, these techniques do not enable the flow velocity of the droplets to be determined. In addition, they require a good refractive index contrast between the two fluids propagating in the micro channel.
The reader may refer to the article by S. A. Leung “Continuous real-time bubble monitoring in microchannels using refractive index detection”, Measurement Science Technology 15, 290 (2004).
The abovementioned techniques therefore do not provide complete information for characterizing the flow.
Several alternative techniques make it possible to obtain more complete data about the droplets or bubbles, especially the size of the droplets, their transit frequency and their flow velocity.
This is for example the case of techniques employing what is called a “capacitive” approach. These techniques make it possible to measure a variation in capacitance due, in the case of a liquid/liquid two-phase flow, to the transit of a droplet of the first liquid in the flow of the other liquid.
However, these techniques require specific micro channels having capacitive sensors. Moreover, they are no longer effective for very small liquid droplets, the variation in capacitance becoming difficult to detect.
The reader may also refer to the article by X. Niu et al., “Real-time detection, control, and sorting of microfluidic droplets”, Biomicrofluidics 1, 044101 (2007) for better understanding the capacitive technique.
This is also the case for techniques based on fiber-optic sensors in the micro channels. In the case of a liquid/liquid flow, these techniques effectively enable the emission frequency of liquid droplets, the flow velocity thereof in the other liquid and the size thereof to be determined.
The techniques based on the use of fiber-optic sensors also require specific micro channels having said sensors.
The reader may for example refer to the article by N. T. Nguyen et al., “Optical detection for droplet size control in microfluidic droplet-based analysis systems”, Sensors and Actuators B 117, 431 (2006) which employs micro channels with fiber-optic sensors.
The most commonly used technique for obtaining complete data consists in using a high-speed video camera for taking images that allows the change in the two-phase flow to be monitored. For example, in the case of a liquid/liquid flow, the video technique makes it possible to measure the size, the velocity and the emission frequency of a droplet of the first liquid in the other liquid.
In addition, the video technique does not need to employ specific micro channels having capacitive or fiber-optic sensors, and therefore can be employed with basic micro channels.
However, the video technique does not make it possible to take measurements in real time since the acquisition images are firstly recorded in a memory and then analyzed by suitable software. The video images taken also consume a large amount of memory space, thereby considerably limiting the capability of tracking the two-phase flow over the course of time.
The reader may for example refer to the article by T. Ward et al., “Microfluidic flow focusing: Drop size and scaling in pressure versus flow-rate driven fluid pumping”, Electrophoresis 26, 3716 (2005).